Allow HotRodDecoder to be shared between Netty pipelines

Currently HotRodDecoder instance can not be shared between Netty pipelines as HotRodEncoder. Currently each pipeline creates its own instance by calling:

server.getDecoder()

in NettyChannelInitializer#initializeChannel. We could probably make it a bit more efficient (or limit allocation rate at least) if we allow sharing single instance between pipelines.